With the development of radio and space technologies, several satellite-based navigation systems (i.e. satellite positioning system or “SPS”) have already been built and more will be in use in the near future. Those systems with full global coverage are sometimes referred to as Global Navigation Satellite Systems (“GNSS”). SPS receivers, such as, for example, receivers using the Global Positioning System (“GPS”), also known as NAVSTAR, have become commonplace. Other examples of SPS and/or GNSS systems include but are not limited to the United States (“U.S.”) Navy Navigation Satellite System (“NNSS”) (also known as TRANSIT), LORAN, Shoran, Decca, TACAN, NAVSTAR, the Russian counterpart to NAVSTAR known as the Global Navigation Satellite System (“GLONASS”), any future Western European SPS such as the proposed “Galileo” program, or any other existing or future system such as the Chinese “Beidou” or “Compass” system. As an example, the U.S. NAVSTAR GPS system is described in GPS Theory and Practice, Fifth ed., revised edition by Hofmann-Wellenhof, Lichtenegger and Collins, Springer-Verlag Wien NewYork, 2001, which is fully incorporated herein by reference.
The U.S. GPS system was built and is operated by the United States Department of Defense. The system uses twenty-four or more satellites orbiting the earth at an altitude of about 11,000 miles with a period of about twelve hours. These satellites are placed in six different orbits such that at any time a minimum of six satellites are visible at any location on the surface of the earth except in the polar region. Each satellite transmits a time and position signal referenced to an atomic clock. A typical GPS receiver locks onto this signal and extracts the data contained in it. Using signals from a sufficient number of satellites, a GPS receiver can calculate its position, velocity, altitude, and time.
In environments where satellite signals are degraded, however, a GPS receiver often encounters problems in locking onto the signals that are needed for the calculation of position, velocity, altitude, and time. In a degraded signal environment, satellite signals can be weak or otherwise difficult for GPS receivers to lock on to. One example of a degraded signal environment is a tunnel through which a car equipped with a GPS receiver is driving. While the GPS receiver is in the tunnel, the satellite signals are completely obstructed and no lock can be acquired. As a result, the GPS receiver cannot accurately calculate its position and other navigational data (such as velocity, altitude, and time) while the car is in the tunnel. In addition, because GPS receivers typically utilize positioning algorithms that use navigational filters that account for the GPS receivers' last-known positions, the GPS receiver's position calculation when the car emerges from the tunnel can also be less accurate since accurate position calculations were not performed during the time that the car was in the tunnel.
Degraded signal environments are often encountered in urban areas, such as cities with many tall buildings. A city with many tall buildings can contain “urban canyons”, which are environments where streets cut through dense blocks of structures such as skyscrapers. In urban canyons, satellite signals are frequently not visible or are degraded due to the signals being partially or fully blocked by buildings, for example. Consequently, the problem of inaccurate position calculations by GPS receivers in degraded signal environments is especially acute in urban areas. Other degraded environments include tunnels, garages, and inside buildings.
Moreover, in difficult signal environments like urban canyons, some direct GPS signals may be blocked by high buildings near the GPS receiver. The reflected signal is called multipath. The pseudoranges affected by the multipath will typically be a summation of the direct (true) pseudorange and some slowly varying biases since the multipath signals have to travel some extra time/distance.
In conventional receivers, the multipath problem is typically addressed by increasing measurement variance in the navigation filter to handle the multipath bias while leaving the bias in the solution. Although this allows the filter to keep operating, this leads to less accurate navigation solutions. Accordingly, a need remains for an effective method for handling multipath.